In the disclosure of the present invention reference is mostly made to drug delivery devices comprising a threaded piston rod, such devices being used e.g. in the treatment of diabetes by delivery of insulin, however, this is only an exemplary use of the present invention.
Drug Injection devices have greatly improved the lives of patients who must self-administer drugs and biological agents. Drug Injection devices may take many forms, including simple disposable devices that are little more than an ampoule with an injection means or they may be durable devices adapted to be used with pre-filled cartridges. Regardless of their form and type, they have proven to be great aids in assisting patients to self-administer injectable drugs and biological agents. They also greatly assist care givers in administering injectable medicines to those incapable of performing self-injections.
Performing the necessary insulin injection at the right time and in the right size is essential for managing diabetes, i.e. compliance with the specified insulin regimen is important. In order to make it possible for medical personnel to determine the effectiveness of a prescribed dosage pattern, diabetes patients are encouraged to keep a log of the size and time of each injection. However, such logs are normally kept in handwritten notebooks, from the logged information may not be easily uploaded to a computer for data processing. Furthermore, as only events, which are noted by the patient, are logged, the note book system requires that the patient remembers to log each injection, if the logged information is to have any value in the treatment of the patient's disease. A missing or erroneous record in the log results in a misleading picture of the injection history and thus a misleading basis for the medical personnel's decision making with respect to future medication. Accordingly, it may be desirable to automate the logging of ejection information from medication delivery systems.
Though some injection devices integrate this monitoring/acquisition mechanism into the device itself, e.g. as disclosed in US 2009/0318865 and WO 2010/052275, most devices of today are without it. The most widely used devices are purely mechanical devices either durable or prefilled. The latter devices are to be discarded after being emptied and so inexpensive that it is not cost-effective to build-in electronic data acquisition functionality in the device itself.
Whereas the above-referred known systems are based on detecting movements generated by the expelling mechanism which then represent translation of the actual expelling structure, i.e. the reservoir piston or the piston rod in direct contact with the piston, it has also been proposed to directly measure the position of the piston. For example, U.S. Pat. No. 5,782,814 discloses a system in which the piston includes a magnetically responsive element, such as an iron core. The system has a receptacle for receiving a syringe reservoir for dose measurement. An inductive element is positioned coaxially to the receptacle to produce a magnetic field. When the syringe is placed in the receptacle, the intensity of the magnetic field varies in dependence upon the position of the piston in the reservoir. The magnetic field induces a voltage in a conducting loop and a voltage meter is connected to the conducting loop to measure the induced voltage. A microprocessor is connected to the voltage meter to calculate the dose from the measurement of the induced voltage. U.S. Pat. No. 6,556,005 discloses a magnetic encoder apparatus capable of determining axial and rotational displacements.
Having regard to the above, it is an object of the present invention to provide systems and methods for reliable and efficient detection of the axial position of an axially moveable element. It is a further object of the invention to provide systems and methods allowing an expelled dose of drug from a drug delivery device to be determined. It is a yet further object to provide means allowing a log for determined values to be created in a safe and efficient way.